A Norwegian coupling or coupler, is a manually operated coupling at each end of some narrow-gauge railway rolling stock. It consists of a central buffer incorporating a hook that drops into a slot in the opposing central buffer. The system is only found on narrow gauge railways with a gauge of 1067 mm or less on which low speeds and train loads allow a simpler system than, for example, knuckle couplers. Norwegian couplings are not particularly strong, and may be supplemented by auxiliary chains. Not all Norwegian couplings are compatible with one another since they vary in height and width. Some may permit a hook from both rail vehicles to be in place; others may be limited to one.